Variations in Mendelian Ratios Explanations for not getting

Variations in Mendelian Ratios Explanations for not getting what you would expect…

Exceptions to Mendel’s Principles • • Linkage Incomplete Dominance Co-Dominance Multiple Alleles Sex-Linked Genes Polygenic Epistasis Pleitropy

Linkage • The characters Mendel examined happened to be on separate chromosomes - that is why he observed independent assortment. • If, however, the genes are on the same chromosomes, they will be inherited together.

Linkage (con’t) • For example, consider the following parental nuclei. Both father and mother have a pair of chromosomes with alleles for two different genes:

Linkage (con’t, still…) • If we look at this with a Punnett square what is going to happen in the next generation:

Linkage (con’t, still…again) • There are fewer genotype combinations than in the usual cross involving two alleles.

Incomplete and Codominance • In incomplete and co-dominance, neither of the two alleles is dominant over the other. • The result is a heterozygote with a different phenotype from the homozygous dominant.

Incomplete Dominance • the effect of the two alleles is blended

Co-Dominance • both alleles are expressed independently and are uniquely recognizable • In white clover, one form of homozygous plant exhibits leaves with a chevron pattern; the alternate homozygous form exhibits leaves with a large light-colored area. • Heterozygous plants exhibit both the chevron and light colored area on their leaves.

Multiple Alleles • Sometimes more than two alleles are present at a locus for a gene. • An example is blood groups in humans.

Multiple Alleles (con’t) • Blood type, for example, is determined by three alleles. – Ia codominant – Ib codominant – i recessive

Bloodtype Alleles Ia Ib i Ia I a. I b I ai Ib I a. I b I bi i I ai I bi ii

Sex-Linked Genes • Alleles for a gene on the X chromosome (but absent on the Y) are said to be sex-linked.

Sex Determination • If two X chromosomes (XX) are present in the embryo, it generally develops into a female. • If one X and Y (XY) are present in the embryo, it generally develops into a male. • Males determine the sex of the offspring as they can create either an X sperm or a Y sperm. • If an X sperm fertilizes an egg, the embryo becomes a female. • Females can only create X eggs.

Back to Sex-Linkage • This means that males may inherit just one allele for a characteristic and that allele will be expressed, whether it is dominate or recessive. • The three genotypes that are present in the female (AA, Aa, aa) are the usual ones. • In males there are two new genotypes (A and a). • Males are said to be hemizygous for sex-linked traits because they only have one X chromosome.

Example Sex-Linkage • Genotypes are expressed with their respective sex chromosome; the Y has no alleles.

Sample Cross (Sex Linkage)

Interactions Between Genes • Independence of gene inheritance does not necessarily mean that genes act independently. • Because genes effect the phenotype via biochemical reactions, their effects depend on the chemical and physical environment created, in part, by other genes. (Understanding this will come later… hopefully!) • Therefore, the phenotypic effect of one gene depends on one or more other genes.

Polygeny • When two or more independent genes have a similar and additive effects on the same characteristic, it is called polygenic inheritance. • Height and skin color in humans, for example, are polygenic.

Epistasis • When the presence of one allele of a gene determines which allele of another gene is expressed. • Coat, eye, and nose color in Labs is a great example

Coat Colour (Phenotype) Allele Genotypes Black B BBEE BBEe Bb. EE Bb. Ee Chocolate b bb. EE bb. Ee Yellow e BBee Bbee bbee • The e allele is epistatic. • When present, it prevents the B or b allele from being expressed and the labs are all yellow.

Pleiotropy • A gene that effects more than one characteristic is pleiotropic. • Sickle Cell is an example.